The present invention relates to a foreign object detection sensor for determining whether there is a foreign object in the proximity of a sensor electrode.
Some conventional vehicles are provided with an electric sliding door apparatus for moving and sliding a door panel (movable body) provided on a side of the vehicle in the front-rear direction of the vehicle using the drive force of a motor or the like. Such sliding door apparatuses are provided with a foreign object detection sensor, such as that described in, for example, Japanese Laid-Open Patent Publication No. 2006-162374, so that foreign objects between the front end portion of the door panel and a peripheral portion of a door opening for passengers to get in and out of the body of the vehicle (opening portion) can be detected during the closing operation of the door panel.
The foreign object detection sensor described in the above described publication has a sensor electrode (antenna), an oscillator, a detection portion, and a determination section. The sensor electrode is placed along the front end portion of the door panel. This sensor electrode detects the capacitance between the sensor electrode and a foreign object in the proximity of the sensor electrode. An oscillating circuit oscillates at an oscillation frequency determined in accordance with the capacitance detected using the sensor electrode. The detection portion detects the above described oscillation frequency. The determination section determines whether there is a foreign object in the proximity of the sensor electrode, that is to say, whether there is a foreign object between the front end portion of the door panel and the peripheral portion of the door opening on the basis of the oscillation frequency detected by the detection portion. Specifically, the determination section determines that there is a foreign object in the proximity of the sensor electrode in the case where the amount of change (or inclination of change) in the oscillation frequency detected by the detection portion exceeds a preset reference value.
The foreign object detection sensor detects the amount of change in the above described oscillation frequency whenever a predetermined period of time elapses or whenever the door panel moves by a predetermined distance. In the case where an amount of change in the oscillation frequency is detected whenever a predetermined period of time elapses, the foreign object detection sensor measures values in the waveform outputted from the oscillation circuit at a first time and a second time within a predetermined period of time, and detects the amount of change in the oscillation frequency on the basis of the results of this measurement. Meanwhile, in the case where the amount of change in the oscillation frequency is detected whenever the door panel moves by a predetermined distance, the foreign object detection sensor generally measures the period in terms of the length of one wave in the waveform outputted from the oscillating circuit when the door panel moves by a predetermined distance, and detects the amount of change in the oscillation frequency on the basis of the results of this measurement.
In the case where the amount of change in the oscillation frequency is detected on the basis of the values in the waveform outputted from the oscillating circuit at the first time and the second time within the predetermined period of time, the number of cycles of the waveform outputted within the predetermined period of time changes. Therefore, there is inconsistency in the precision of the amount of change in the oscillation frequency gained for the predetermined period of time. The oscillation frequency in the oscillating circuit changes in accordance with the change in the capacitance detected using the sensor electrode, and therefore, in some cases, the number of cycles in the waveform outputted from the oscillating circuit within a predetermined period of time does not become a natural number. In such cases, the amount of change in the oscillation frequency gained for every predetermined period of time ends up having low precision.
Meanwhile, in accordance with the method for measuring the cycle in terms of the length of one wave in the waveform outputted from the oscillating circuit whenever the door panel moves by a predetermined distance, there is a risk that there may be inconsistency between the result of measurement and the actual value in the case of a measuring method with low precision.
As described above, there is a risk in the foreign object detection sensor described in Patent Document 1 that the precision in detection of the amount of change in the oscillation frequency on the basis of the waveform outputted from the oscillating circuit may be low. That is to say, there is a risk that the precision in the detection of the capacitance by the sensor electrode may be low, and the precision in the detection of foreign objects in the proximity of the sensor electrode may be low.